1. Field of the Invention
The present invention consists in a satellite for a telecommunication system. It also consists in a method of dynamic compensation of signals for a satellite of the above kind.
2. Description of the Prior Art
The invention concerns satellite telecommunications and more precisely space telecommunication system repeaters. In space telecommunication systems signals transmitted by satellites or repeaters to earth stations are attenuated differently and received with greatly different powers. The higher the frequency the greater the difference, which can be as high as 20 dB at 30 GHz when there is high local precipitation.
Equipment cost constraints, and in particular those affecting mobile equipment for satellite telecommunication systems, lead to a reduction in the capacities of the equipment; the equipment is then unable to compensate power losses of this magnitude. This is particularly critical for future satellite systems operating in the Ka band; for such systems the coverage of a given satellite is much larger than the typical dimension of such climatic phenomena. There therefore coexist in the downlink transmission channel signals having differences of level that can be as high as 20 dB, for similar data bit rates. Power differences of this magnitude make spurious coupling phenomena critical and accentuate the impact of amplifier non-linearities.
Attenuation problems of the above kind also arise on the uplink connections, on which level differences of this magnitude can occur.
EP-A-0 289 130 describes the problem of intermodulation products between different frequencies of a radio transmitter caused by non-linearities of the power amplifier; it proposes the provision of a series of predistortion circuits, a chosen circuit being connected downstream of the mixer device of the transmitter and upstream of the power amplifier and the antenna. The predistortion circuit used is selected according to the frequency of the signal to be transmitted and induces in the signals distortion opposite to that induced by the amplifier for the frequency range concerned. This reduces intermodulation products between the different frequencies to levels below 40 dB. The above document suggests only precompensation varying in accordance with the frequency to assure linear amplification of the signal throughout the wanted band.
Compensation of the kind proposed in EP-A-0 289 130 is insufficient in the case of a space telecommunication repeater in that at best it can compensate attenuation differences on the uplink transmission part. The same problems of link attenuation arise for the downlink part: a signal transmitted in an area with heavy precipitation is more attenuated, which increases the dynamic range of the signal received by the terrestrial equipment.
Document EP 0 805 568 describes a power control method and device for limiting the dynamic range of the signal received by the terrestrial equipment. To this end the ground equipment includes a control loop acting on the entire system. This configuration has the advantage of providing precise results in terms of limiting the dynamic range of the signal received by the terrestrial equipment but the disadvantage of using a complex structure requiring considerable modification of the ground equipment without having a transparent action on the signals transmitted.
The invention proposes a solution to the problem of fluctuations in the attenuation of different signals in a space telecommunication system; it proposes a solution that not only compensates the attenuation differences on the uplink transmission part but also precompensates attenuation differences on the downlink transmission part, without modifying the ground equipment and without interfering with the architecture of the satellite.
To be more precise, the invention proposes a satellite for a satellite telecommunication system including means for transmitting signals at different powers on the downlink connection enabling transparent digital processing of the signals in the satellites so that the powers are chosen to limit the relative dynamic range of the signals received on the downlink connection.
xe2x80x9cTransparent processingxe2x80x9d means processing that does not modify the waveform of the signal and which is therefore transparent with regard to information transmitted by the satellite. In particular, transparent processing does not include any encoding or decoding, modulation or demodulation step.
The relative dynamic range of the signals received on the downlink connection is advantageously less than 5 dB and preferably less than 2 dB.
In one embodiment, the transmitter means comprise at least two amplifiers and means for routing signals to the amplifiers in accordance with the transmitter power to be applied to the signals.
The satellite preferably includes means for compensating power differences of signals applied to a given amplifier.
In one embodiment, the transmitter means comprise an analog-digital converter, fast Fourier transform means for converting the converted signals, signal gain control means, means for routing signals to at least two amplifier subsystems each including Inverse fast Fourier transform means, a digital-analog converter and a power amplifier.
The invention also proposes a method of dynamically compensating signals for space telecommunication transparent digital repeaters, the method comprising a step of separating signals received into at least two groups and a step of transmitting signals of the groups at different powers according to the group, the powers being chosen to limit the relative dynamic range of signals received on the downlink connection.
The relative dynamic range of signals received on the downlink connection is advantageously less than 5 dB and preferably less than 2 dB.
In one embodiment, the step of transmitting signals of a group includes a step of equalizing the powers of the signals of the group and a step of amplifying the signals.